This application is based on French Patent Application No. 00 12 725 filed Oct. 5, 2000, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The present invention relates to a terminal adapted to be powered locally and to receive a remote power feed via a link connecting it to a computer local area network, for example an Ethernet network.
2. Description of the Prior Art
Terminals connected to a computer local area network (for example personal computers, printers, etc.) are conventionally powered locally, from the mains power supply. 110 V or 220 V power cords independent of the data connections therefore power the terminals. This solution increases the difficulty of installing the local area network:
The use of two cords gives rise to problems of overall size that can additionally restrict the movement of persons.
It creates electrical hazards.
A telephone powered locally by the mains power supply also has the disadvantage of being out of service in the event of a mains power outage, in particular in the case of fire or natural disaster. This is why conventional telephones receive a remote power feed from their local exchange, which includes back-up batteries.
Telephone terminals connected to a computer local area network are coming into use. It is therefore desirable for some terminals connected to a computer local area network to receive a remote power feed over the same link as that used to send/receive data. One way to transmit a remote power feed current is to use two of the eight wires of the link: four other wires of the eight constitute two pairs of wires respectively used to transmit and receive data. Another method, known as a phantom circuit, connects the two terminals of a power supply generator in the remote power feed device to respective center-taps of a transformer winding connected to the pair for receiving data and a winding of another transformer connected to the pair for sending data. At the terminal end, the power supply voltage is supplied by respective center-taps of a transformer winding connected to the pair for receiving data and a winding of another transformer connected to the pair for sending data.
In both cases, providing a remote power feed to a terminal via the computer local area network has the disadvantage that the remote power feed device powers a terminal blind. The RJ45 connector at the end of the link may be connected to a terminal of a type other than a telephone (for example a personal computer, a printer, etc.). There is a risk of damaging the electrical circuits of the terminal. The RJ45 connector of a terminal is generally used in the following manner:
Four of the eight wires are separated into two pairs for respectively transmitting and receiving data. The terminal includes a transformer having a winding connected to the receive pair and a transformer having a winding connected to the send pair, each of these windings having a center-tap that can be connected to a reference potential via a resistor of low resistance.
Four other wires, not used, are grounded, often via a combination of resistors and capacitors, to eliminate any crosstalk currents induced by data signals in the first four wires and to reduce undesirable electromagnetic emission. If a relatively high power supply voltage, for example 48 volts, is applied to this combination of resistors and capacitors, or to the resistors connected to the center-taps of the transformers, the current flowing in the resistors can destroy them.
Methods are currently under investigation for providing a remote power feed to a terminal in a computer local area network preventing all risk of damage if a terminal that is not adapted to receive a remote power feed via the network is connected to the network.
This kind of method consists of:
producing a test signal, or a plurality of test signals, on at least two conductors of a link for connecting the local area network to a remote terminal, the energy of the signal(s) being such that the terminal cannot be damaged under any circumstances;
detecting the presence of a remote terminal adapted to receive a remote power feed by detecting the presence of identification modules shunting the link in the remote terminal, on the basis of the current produced by the test signal(s) in the link; and
sending a remote power feed current over the link if and only if the presence of a terminal adapted to receive a remote power feed has been detected.
This kind of method prevents all risk to the terminals because the remote power feed current is sent only after identifying the terminal as one adapted to receive a remote power feed. The current and the duration of the test signal or signals are chosen so that the operation of detecting the presence of a terminal adapted to receive a remote power feed before providing the remote power feed cannot cause any damage if the terminal is not a terminal adapted to receive a remote power feed.
Also, the remote power feed must be interrupted as soon as the terminal is disconnected from the link, because another could be connected at any time. There is therefore provision for continuously detecting the presence of a terminal during remote power feeding, by measuring the remote power feed current. The remote power feed device concludes that there has been a disconnection when it detects that the current drawn is less than a predetermined threshold value during a time interval having a predetermined duration sufficient for it to be sure that there has really been a disconnection.
The following are used in the same computer local area network: terminals powered only locally, terminals powered only by a remote power feed, and terminals powered locally in normal operation but adapted to receive a remote power feed in the event of failure of the local power supply, at least to maintain basic functions.
This latter type of terminal is provided with an identification module similar to that of a terminal that must receive a remote power feed at all times, but obviously does not draw any remote power feed current since it is powered locally. For the remote power feed device, the absence of a remote power feed current would make it resemble a disconnected terminal. This absence of a remote power feed current would lead the remote power feed device to suspend the remote power feed over the link. The local power supply would therefore have no back-up. One feasible solution to this problem would be to provide in the terminal a resistor shunting the two remote power feed poles and having a resistance such that the current drawn continuously from the remote power feed would be above the threshold value. However, this solution would have two drawbacks:
A non-negligible reduction in the maximum power that the remote power feed could provide to this kind of terminal when backing up the local power supply: if the threshold value is 20 mA and if the nominal voltage of the remote power feed is 48 volts, for example, the power dissipated unnecessarily is approximately 1 W per terminal. If the maximum power that can be delivered via the link is 14 W, for example, the maximum power effectively available for the remote power feed to the terminal is only 13 W, which reduces the capabilities of the terminal.
Greater difficulty in implementing the identification module of the terminal adapted to receive a remote power feed, since the additional resistor shunts the identification module, in particular during the tests to identify the type of terminal. It therefore modifies the apparent electrical characteristics of the module. Producing an identification module operating satisfactorily is therefore more complicated.
A first object of the invention is to enable better use of the power that the remote power feed can supply when it is backing up the local power supply of a terminal.
The invention provides a terminal adapted to be powered locally and to receive a remote power feed via a link connecting it to a local area network, the terminal including a local power supply and an identification module connected to at least two conductors of the link, the identification module enabling remote detection that the terminal is adapted to receive a remote power supply, which terminal further includes:
means for drawing a predetermined remote power feed current, and
control means including means for activating the current-drawing means when the local power supply is working normally and for inhibiting the current-drawing means when the local power supply is not working normally.
When the local power supply cannot power it, the above terminal has available to it all of the maximum power that the remote power feed can supply, because the control means inhibit the means for drawing a predetermined remote power feed current until they register the fact that the local power supply is again able to power the terminal.
When the local power supply is again able to power it, the above terminal again draws a predetermined remote power feed current chosen to be sufficient for the terminal to be detected by the remote power feed device as being still connected.
In a preferred embodiment, the control means further include means for inhibiting the current-drawing means when the remote power feed is not applied to the link. Thus the current-drawing means do not affect the operation of the identification module during the process of detecting that the terminal is adapted to receive a remote power feed, which precedes the application of the remote power feed.
The invention will be better understood and other features of the invention will become apparent in the course of the following description and from the accompanying drawings.